Morphological changes during ontogeny of the canine proximal colon

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Abstract
The development of the canine proximal colon from the completion of organogenesis through 43 days after birth was studied using light microscopy, immunofluorescence and electron microscopy. During this period the tunica muscularis increased in thickness from 42±6 μm in animals midway through the gestation period to 317±29 μm in animals 25–30 days old. This increase in thickness resulted from an increase in the number and size of smooth muscle cells in the circular and longitudinal muscle layers. The cross-sectional thickness of the circular muscle layer increased from 10±2 smooth muscle cells midway through the gestation period to 92±7 cells in animals 25–30 days old. The longitudinal layer increased in thickness from 1.5±1 cells in animals midway through the gestation period to 44±2 cells in animals 25–30 days old. Smooth muscle cells from both layers also increased in diameter and length. Ultrastructural and immunohistochemical studies suggested that many of the smooth muscle cells were undergoing development throughout the fetal period. Midway through the gestation period, the circular layer was positive for desmin-like immunoreactivity (D-LI), while both the circular and longitudinal layers were positive for vimentinlike immunoreactivity (V-LI). By birth, V-LI was suppressed in the circular and longitudinal layers, and both layers expressed D-LI. The enteric nervous system was already established midway through the gestation period, and submucosal and myenteric ganglia could be identified, although the chemical coding and mature morphology of neurons were incomplete. NADPH-diaphorase-positive neurons, indicating the expression of nitric oxide synthase, developed by the time of birth. Interstitial cells of Cajal (IC) could not clearly be identified midway through gestation, however, potential precursors to ICs were observed. Several classes of ICs were identifiable at birth.